How fossil fuels formed

When I was quite young, someone told me that all the fuel that exists in the form of oil and coal came from long dead dinosaurs. What amazes me is that I never questioned this preposterous piece of information for a long time. I think the fact that they were referred to as ‘fossil fuels’ confirmed in my mind that dead animals were the source and dinosaurs, being the largest dead animals, seemed to fit the bill.

It was only much later that the thought occurred to me that given the amount of oil and coal that exists and that we have already extracted, there must have been a hell of a lot of dinosaurs to produce it and that was just not possible, and that while dinosaurs were big, they could not be the sole source. This informative page from the US Department of Energy suggests that I was not alone in my goofy misapprehension, so I thought that I would spread the word.

Contrary to what many people believe, fossil fuels are not the remains of dead dinosaurs. In fact, most of the fossil fuels we find today were formed millions of years before the first dinosaurs.

Fossil fuels, however, were once alive!

They were formed from prehistoric plants and animals that lived hundreds of millions of years ago.

Think about what the Earth must have looked like 300 million years or so ago. The land masses we live on today were just forming. There were swamps and bogs everywhere. The climate was warmer. Ancient trees and plants grew everywhere. Strange looking animals walked on the land, and just as weird looking fish swam in the rivers and seas. Tiny one-celled organisms called protoplankton floated in the ocean.

When these ancient living things died, they decomposed and became buried under layers and layers of mud, rock, and sand. Eventually, hundreds and sometimes thousands of feet of earth covered them. In some areas, the decomposing materials were covered by ancient seas, then the seas dried up and receded.

During the millions of years that passed, the dead plants and animals slowly decomposed into organic materials and formed fossil fuels. Different types of fossil fuels were formed depending on what combination of animal and plant debris was present, how long the material was buried, and what conditions of temperature and pressure existed when they were decomposing.

For example, oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and “cook” the organic material under layers of silt. In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed. Over time, some of this oil and natural gas began working its way upward through the earth’s crust until they ran into rock formations called “caprocks” that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today.

The same types of forces also created coal, but there are a few differences. Coal formed from the dead remains of trees, ferns and other plants that lived 300 to 400 million years ago.

This is what I like so much about science, that being wrong is the precursor to finding out interesting new things. But it requires being willing to look a little below the surface of folklore and not accept the glib answers that are often proffered for questions about our world.